cmake: use MSVC_RUNTIME_LIBRARY to force MT

CMakeLists.txt

@@ -2,6 +2,9 @@ if(${CMAKE_CURRENT_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_BINARY_DIR})
   message(FATAL_ERROR "Prevented in-tree build. Please create a build directory outside of the SDL source code and run \"cmake -S ${CMAKE_SOURCE_DIR} -B .\" from there")
 endif()
 
+# MSVC runtime library flags are selected by an abstraction.
+set(CMAKE_POLICY_DEFAULT_CMP0091 NEW)
+
 cmake_minimum_required(VERSION 3.0.0...3.5)
 project(SDL2 C CXX)
 
@@ -253,6 +256,9 @@ endif()
 if(MSVC)
   option(SDL_FORCE_STATIC_VCRT "Force /MT for static VC runtimes" OFF)
   if(SDL_FORCE_STATIC_VCRT)
+    if(NOT DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
+      set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+    endif()
     foreach(flag_var
         CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
         CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO)